Vectorization vs loops in MATLAB - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of vectorized vs loop sum

What is the output of the following MATLAB code snippet?

MATLAB

A = 1:5;
loop_sum = 0;
for i = 1:length(A)
    loop_sum = loop_sum + A(i);
end
vector_sum = sum(A);
disp([loop_sum, vector_sum]);

A [15 15]

B [10 15]

C [15 10]

D [0 15]

💡 Hint

Think about how sum and loops accumulate values.

Both the loop and the vectorized sum compute the sum of elements 1 through 5, which is 15.

❓ Predict Output

intermediate

Result of element-wise multiplication

What is the output of this MATLAB code?

MATLAB

A = [1 2 3];
B = [4 5 6];
for i = 1:length(A)
    C(i) = A(i) * B(i);
end
disp(C);

A [1 2 3]

B [5 7 9]

C [24 30 36]

D [4 10 18]

💡 Hint

Multiply each pair of elements at the same position.

Each element of C is the product of corresponding elements in A and B: 1*4=4, 2*5=10, 3*6=18.

❓ Predict Output

advanced

Vectorized vs loop matrix multiplication output

What is the output of this MATLAB code?

MATLAB

A = [1 2; 3 4];
B = [5 6; 7 8];
C = zeros(2);
for i = 1:2
    for j = 1:2
        for k = 1:2
            C(i,j) = C(i,j) + A(i,k)*B(k,j);
        end
    end
end
D = A*B;
disp(C);
disp(D);

A

[[19 22]; [43 50]]
[[19 22]; [43 50]]

B

[[17 20]; [39 46]]
[[19 22]; [43 50]]

C

[[19 22]; [43 50]]
[[17 20]; [39 46]]

D

[[0 0]; [0 0]]
[[19 22]; [43 50]]

💡 Hint

Matrix multiplication sums products of rows and columns.

Both the loop and the vectorized multiplication produce the same matrix [[19 22]; [43 50]].

❓ Predict Output

advanced

Performance difference in vectorized vs loop sum

Which statement about the output of this MATLAB code is true?

MATLAB

N = 1e7;
A = ones(1,N);

% Timing loop sum
tic
s1 = 0;
for i = 1:N
    s1 = s1 + A(i);
end
t_loop = toc;

% Timing vectorized sum
tic
s2 = sum(A);
t_vec = toc;

fprintf('Loop time: %.4f, Vector time: %.4f\n', t_loop, t_vec);

A Loop sum is significantly faster than vectorized sum.

B Both times are about the same.

C Vectorized sum is significantly faster than loop sum.

D Code will produce an error due to large N.

💡 Hint

Think about how MATLAB optimizes vector operations.

Vectorized operations in MATLAB are optimized and run faster than explicit loops for large arrays.

🧠 Conceptual

expert

Why vectorization is preferred over loops in MATLAB?

Which of the following is the main reason MATLAB users prefer vectorized code over loops?

A Vectorized code is easier to read but slower to execute.

B Vectorized code runs faster because MATLAB is optimized for matrix and vector operations.

C Loops are deprecated in MATLAB and cannot be used anymore.

D Loops cause syntax errors in MATLAB.

💡 Hint

Think about MATLAB's design focus on matrices and vectors.

MATLAB is designed to efficiently handle vector and matrix operations internally, making vectorized code faster.